1. Field of the Invention
The present invention relates to a video signal processing device, hereinafter referred to as the invention or TIBEC (time base equalizer and/or corrector), that equalizes or corrects the timing errors generated by video tape or disk machines operating in playback mode.
2. Description of the prior art
Present-day helical-scan video tape machines or video disk playback machines, when operated in playback mode, reproduce a video signal (hereinafter referred to as INCORRECT video), that exhibits two peculiar--and non standard--characteristics: jitter and incoherence.
a. "INCORRECT" Video Definition.
i. Jitter and Cummulative Jitter.
The horizontal timing of the playback video exhibits a significant degree of instability, hereinafter referred to as JITTER. The instability is well outside the limits stated in EIA standard RS-170-A. Furthermore, the jitter is cumulative, i.e., the duration of consecutive played-back horizontal lines may be longer (or shorter) than the duration for consecutive standard lines, so the accumulated time deviation of the VTR signal as compared with a standard signal may reach several horizontal (H) intervals at the peak of one cycle of accumulation.
The jitter is mostly caused by the inability of the VTR's to control the rotation speed of the drum servos when on playback, and keep it constant or, at the least, identical to the drum rotation speed during recording. The effect is further aggravated when recording or playback takes place while the machine is rotating with vector components of its angular speed perpendicular to the drum shaft. Gyroscopic torques are created by the combination of its angular momentum and machine rotation, and these torques, plus the increased shaft friction torques, can not be compensated by the drum servos and drive motors. Since a full video frame, consisting of 262.5 lines of video is recorded on each semirevolution of the drum, it is quite obvious that these speed differences result in an inherent jitter of the playback video signal. The recording and playback timing errors are directly proportional to the drum speed differences when recording and playing back the same video frame. The playback H interval elongates if the speed difference is positive, and shortens if the difference is positive. The speed difference is largely random, and when one of the two conditions lasts for periods of several H lines, accumulated jitter is the inevitable result.
ii. Incoherence.
The phase relationship between the horizontal sync and the color subcarrier burst of the playback signal is completely lost, even if a correctly phased signal is used for recording. This condition, by itself and separately from the jitter, will be referred to in this report as incoherence.
Incoherence is the result of bandwidth compression required for recording broadband video signals in magnetic tape using small amounts of tape length, as conventionally done in heterodyne VTR's. On these machines, the chroma and luma contents of the video signal to be recorded are first separated. The signals are modulated on separate carrier frequencies and then combined and recorded by the same heads. In most "color under" machines, the chroma carrier frequency is lower than the luma carrier, due to the smaller bandwidth of the chroma information. At playback, both signals are reproduced together and then separated by frequency. The chroma is remodulated over a stable 3.58 MHz. carrier by heterodyne process, while the luma is just demodulated from its carrier. Luma and chroma are then mixed, resynced with a subcarrier burst derived from the stable 3.58 MHz. oscillator and then sent to the output terminal of the VTR. The result of such process inherently destroys the phase relationship between the horizontal sync,--that is recovered from the luma playback signals,--and the color subcarrier, that is regenerated from the stable 3.58 MHz. oscillator after the hetrodyne process takes place and, consequently, is independent of the luma timing.
iii. Summary of Incorrect Video Definition.
In summary, and using the terminology just defined in the above paragraphs, a VTR playback video signal is INCORRECT video, i.e., it has JITTER and/or it is INCOHERENT.
Incorrect Video in TV Sets and Monitors.
Television receivers, video monitors and VTR's are indeed capable of extracting the luma and chroma information of the incorrect video. Of course, such capability is the fundamental design criterium of any VTR, and lays at the base of the acceptance and widespread use of these machines.
The Slave Video timing errors are CORRECTED when a correctly timed video signal is applied to the Master Video input of the device. The expression "correct timing", as used in this document, refers to a video signal that complies with the National Television Advisory Commitee (NTSC) standard, as further defined by the Electronics Industries of America (EIA) Tentative Standard RS-170-A of Nov. 8, 1977, or other video standards such as PAL, SECAM etc. in use in countries other than the U.S.A.
Furthermore, the present invention also permits to alter the timing errors of the Slave video playback machine, and make them EQUALIZE, or "make them equal" to the timing errors of the incorrect video signal created by a free-running video tape machine or a standard, hereinafter Master video, connected to the Master Video input of the device.
This report refers to the NTSC video standard used in the U.S. and in other countries including Japan. However, the field of the present invention includes the same or similar circuits and claims applied to other television standards, such as PAL and SECAM, and is applicable to any other future video standard such as the emerging High Definition TV (HDTV).
Incorrect Video and Special Effects Generators.
The two characteristics of incorrect video make VTR playback video absolutely useless for the purpose of being combined or mixed in Special Effects Generators (S.E.G.'s), like Scitech's Model HS-3.
For the purpose of this application, "Special Effect" means the combination of two or more video signals sharing the same video line and/or video frame. Split Screens, Wipes, Dissolves etc. are examples of such Special Effects. Single-video effects, such as slow or fast motion, freeze frame and others, even if they are often referred to in the trade literature as Special Effects, they are not included in this definition.
For the generation of Special Effects, the horizontal timing and the color subcarrier phase of the video signals from both sources must be kept identical at all times. This assertion becomes obvious observing that the combined Special Effect Video shares the same H scan line, and, consequently, is decoded in relation to a single horizontal sync pulse for luma and a single subcarrier burst for the chroma.
Time Base Correction and Equalization.
Identical H timing and subcarrier phase does not mean that both videos must be referred to CORRECT parameters. For the purpose of Special Effects generation, the time base of both video signals involved must be equal, but not necessarily correct, or equal to a correct standard. If both signals are incorrect, but their H timing and subcarrier reference phase are equal, both signals can be used to generate Special Effects. The last condition is referred to in this report as `equalized` videos.
As a separate consideration, VTR uncorrected playback signals do not comply with the NTSC standards mandated by the Federal Communication Commission for broadcast video signals. Consequently, they can not be legally aired without previous correction of its time base.
Prior Art TBC's: Correction Window.
Prior art Time Base Correctors (TBC's) correct the playback signals of slaved VTR's by temporarily storing a "window", consisting of a number of H lines, (usually 16 or more), and replaying the stored video with the proper velocity changes to restore a correct time base. The luma and chroma contents of the VTR video is separated before the storage process, that must be carried out by separate and duplicate circuitry for the luma and chroma subsignals.
The window of correction must be large enough to correct for the largest CUMMULATIVE error existent in the video signal. The storage process may be analog or digital.
Prior Art Dual TBC's Requirement.
For the generation of Special Effects, the TBC process must be applied to BOTH VTR's. Indeed, the two corrected signals are made equal to the same standard, and hence, are equal to each other. Equalization is thus achieved by correcting both signals to the same standard.
Several technical papers state that it is impossible to "track" the time base error of one VTR with another VTR. In the papers known by the inventors, no reason is given to support such assertion. The present invention does just that allegedly "impossible" task.
The Costly and Wasteful Dual TBC Requirement.
As mentioned before, re-recording from one VTR to another without time base correction is common practice, both in commercial tape reproduction and single source editing, as well as in consumer tape duplication.
The use of TWO TBC's for A/B roll--double source, namely: A and B--editing, as required by present art TBC's, is costly and wasteful. Costly, because two TBC's are required to correct the output of BOTH, A and B VTR's, and today's TBC's are expensive.
Wasteful, because the usual destination of the output of the two TBC's and Special Effect Generator (S.E.G.) is usually a third VTR used in recording mode. When the corrected and mixed signal is played back by the same or other VTR, the mixed signal will be played back as INCORRECT video again.